1. Field of the Invention
The present invention relates to a circuit for providing a second output voltage derived from a tapped connection to the storage inductor of a first output voltage in a switching power supply converter.
2. Description of the Related Art
Computer systems are fast becoming smaller and more powerful. Although laptop computers still serve many needs, the smaller and more efficient notebook computers are becoming more popular since they are more convenient to use in the business environment. To achieve the reduction in size and increase in efficiency, computer designers of notebook computer systems must reduce power consumption in as many areas as possible. It has been recognized that one significant area to reduce the power consumption is to lower the voltage levels required to drive the logic and memory circuits of the computer system.
Most modern computer systems use logic circuits which require a 5 volt power supply for proper operation. Logic circuits are now available which operate with 3.3 volt power supplies, with the 3.3 voltage level quickly becoming a new standard in the industry. The lower 3.3 volt supply offers the advantages of reduced power consumption which significantly increases efficiency.
Many of the logic circuits operating on 3.3 volts, however, are much more expensive than their 5 volt counterparts, so that it is not necessarily cost effective to operate all of the logic systems with a single 3.3 volt power supply. For example, 3.3 volt dynamic random access memories (DRAMs) are available, but are very expensive. Additionally, while certain major components are available in 3.3 V versions, such as the microprocessor and the memory devices, many other components are not available in 3.3 V versions and conversion of these parts will not occur for some time. It is desirable therefore to have a hybrid system where a portion of the computer is operated from a 3.3 volt power supply, while the remaining portions of the computer are operated from a 5 volt source. A significant disadvantage with this type of hybrid computer system is that the power supply must now provide two separate output voltages, which effectively increases the size and complexity of the power supply. Since the 5 volt and 3.3 volt outputs are providing power for logic systems which generate signals with information content, these outputs must be relatively accurate.
In typical dual output voltage power supply converters of the prior art, one output is regulated while the other output is slaved to the regulated output by some second order effect such as transformer coupling. This method causes the slaved output to be less accurate than the regulated or primary output. The slaved output can be made more accurate by series regulation. Series regulation by means of a linear dissipative element, however, is inefficient. Series regulation with a magnetic amplifier, or an additional pass switched coupled with an inductor/capacitor (LC) filter is costly and bulky.
It is therefore desirable to provide a relatively efficient power supply converter having two separate and relatively accurate output voltages to drive the respective logic circuits, without significantly increasing the cost and size of the power supply.